In a typical existing full matrix capture (FMC) acquisition, a plurality of pulser elements of one or more ultrasonic array probes are individually pulsed and an A-scan (a plot of echo response amplitude vs reception time) is received for each pulse at each of a plurality of receiver elements. The result of the acquisition is an N×M matrix of response A-scans, where M is the number of pulsers and N is the number of receivers. The existing FMC acquisition method enables full beam forming capabilities in post-processing for both emission and reception. Of all the possibilities enabled by FMC, one of the most promising aspects is the ability to provide optimized focusing at all of the imaging plane positions. This is referred to as Total Focusing Method (TFM).
In existing practice, TFM is applied by dividing the imaging volume into an array of voxels, and summing the response A-scans from the FMC matrix, with delays appropriate to the time-of-flight from each pulser via each voxel to each receiver. The term “voxel” is used herein to denote an elementary volume within the imaging volume, analogous to the term “pixel” as applied to two-dimensional images.
Since the calculations performed on FMC data to achieve a TFM image involve determining time-of-flight, the acoustic velocity of the relevant wave type in the test object must be known. Relevant waves types are shear waves (hereinafter referred to as S-waves) and longitudinal waves (hereinafter referred to as P-waves). A significant problem in FMC/TFM analysis is that the acoustic velocity in steel, for example, depends on the composition of the test object material, its thermal treatment and other factors that are not known when doing a non-destructive inspection. In fact, as shown in FIGS. 1A and 1B, the imaging results are extremely sensitive to the assumed sound velocity. FIGS. 1A and 1B show TFM images for a weld inspection using FMC-TFM angle beam with two probes. The image of FIG. 1A was calculated assuming a S-wave velocity of 3,240 m/s and the image of FIG. 1B was calculated assuming a S-wave velocity of 3,320 m/s. Note that there are significant differences in the images, particularly with respect to the image intensity in the vicinity of the indication.
In current practice, ultrasonic velocity measurement methods are mainly based on calculating the time necessary to reach a reflector at a given distance from the probe. However, all methods in current practice involve use of calibration blocks which by definition are not exactly the same material as the test object. Therefore the velocity measurements are necessarily imprecise with respect to the test object, and the quality of the resulting TFM images is significantly affected.
When using two probes in “pitch-catch” (P-C) mode for FMC-TFM imaging, the time-of-flight also depends on the distance between the probes. Therefore the TFM images will also sensitively depend on accurate knowledge of that distance. Typically the probe distance is maintained by means of a mechanical link whose length is adjustable. Therefore it is important to have a measurement method which can confirm that the length of adjustable link has not been inadvertently changed, and that the defined probe distance is effectively maintained throughout a lengthy series of inspections.
There therefore exists a need for a method of accurately determining the acoustic velocity in the test object during the inspection so that the TFM images resulting from the inspection will provide a reliable measure of the intensity of indications.
Furthermore, there also exists a need for a method of accurately determining the distance between probes during a P-C FMC-TFM inspection.